Plastic cards are typically made by lamination of films, which are adapted as polymer layers. For this purpose, a close connection between the polymer layers is produced under pressure at a temperature, which is above the softening point (glass point) of the polymer layers, by that the polymer chains being movable above the glass point mix with each other, and in an ideal case an inseparable, monolithic structure is obtained from the polymer layers.
These polymer layers may in part be provided with additional features, e.g. pigmentations for optimizing the densities are included in laser engraving.
Between the polymer layers, additional elements are normally provided, in particular an intermediate layer, for instance a printing layer in the form of security printing elements, such as guilloches, microprint and so on or special inks such as optical variable ink (OVI) and the like may be provided on one of the polymer layers and in the completed structure between the polymer layers. An intermediate layer may however also be for instance a film with diffractive elements as a security feature. In particular it is also possible, to integrate a photographically produced image as an intermediate layer between the polymer layers. A method with regard thereto is described in the document DE 10 2007 018 450.8. Such an intermediate layer may cover or underlie a part only of the underlying and/or overlying polymer layer, may however also be stacked therewith in a coincident manner.
It is common to all these examples that the intermediate layer may disturb the above process of the lamination, in particular if the intermediate layer is incompatible with the polymer layers to be combined. This incompatibility may consist in that the intermediate layer obstructs as a separating layer the process of the mixture of the softened polymer layers.
For instance, it has been found that printing layers will normally excellently adhere to the actual printing substrate, the first polymer layer, in the completed structure, the adhesion to the second polymer layer applied during the lamination on the printing layer is worse compared thereto, and consequently a delamination may occur between the intermediate layer and the second polymer layer. The weak point is thus the boundary layer between the printing layer and the applied and laminated second polymer layer. This may be particularly critical for full-surface printing on the first polymer layer.
From the document EP 0 688 839 A2, polycarbonates based on a geminally disubstituted dihydroxydiphenyl cycloalkane are per se known. In this prior art, such polycarbonates are used as binding agents of silk-screen printing inks. From this document can also be taken methods for making such polycarbonates. This document with its complete contents is hereby included in the scope of disclosure of the present application.